1. Field of the Invention
This invention relates to the field of mechanical fasteners and devices for their removal. More specifically, the invention comprises a screw extractor configured to remove screws such as pedicle screws and similar orthopaedic hardware. Although the invention was developed for use in the field of medicine, those skilled in the art will realize that it has application in many other fields.
2. Description of the Related Art
The field of orthopaedic medicine involves the application of plates and screws to stabilize compromised articulations in the human body. One typical application is stabilization of the lumbar vertebrae after collapse of one or more intervertebral disks. FIG. 1 is an illustration of two lumbar vertebrae 10. Intervertebral disk 11 lies between the two. In this example, instability of the spine has necessitated the joining of the two vertebrae.
It is well known in the art that vertebrae can be joined by bone grafting. However, it is generally necessary to stabilize the position of the two vertebrae for some period in order to allow the joining to occur. Accordingly, plate 12 is attached to the two vertebrae. Typically two plates 12 would be applied, with one lying on each side of the posterior spinous processes. Each plate 12 is pierced by a set of holes, through which pedicle screws 14 are inserted.
In order to install pedicle screws 14, holes must be drilled through the pedicle portions of the two vertebrae. Pedicle screws 14 are then threaded into these holes and tightened. If all goes well, the two vertebrae will eventually fuse together, thereby eliminating any articulation at the joint. The fusing will ideally render the patient asymptomatic, though obviously somewhat less flexible.
The ideal result is not always achieved, however. Those skilled in the art will know that great variations exist in human anatomy. They will also know that the surgeon is unable to fully visualize the structures involved. These factors may lead to imperfect results.
The reader will observe in FIG. 1 that nerve root 20 exits the foramen between the two vertebrae in a position which is close to the lower pedicle screw 14. A portion of this pedicle screw 14 may protrude beyond the surface of the lower vertebra 10 (As the vertebra comprises a highly irregular shape, the hole drilled therethrough may intersect the outer surface at one or more points. A portion of the threaded pedicle screw shaft may thereby be exposed). This portion may rest against nerve root 20, possibly even compressing nerve root 20. In such an event, the patient may experience common neurological symptoms, such as pain or numbness.
After the joint has stabilized, it may be desirable to remove pedicle screw 14 (as well as possibly plate 12). This operation has typically been performed under general anesthesia. A substantial incision is made through the skin 18, and fascia 16, to reveal the spinal column. The muscles and other structures attached to the posterior spinous processes must then be removed in order to expose pedicle screw 14. A wrench is then used to back pedicle screw 14 out of the bone and remove it through the open incision.
It is often difficult to determine whether a particular pedicle screw is the source of the neurological symptoms experienced by the patient. As the patient is asleep during the procedure, there is no way to determine if the removal of the screw altered the symptoms until much later.
FIG. 2 shows another common factor in the placement of such hardware. The reader will observe that pedicle screw 14 is angularly displaced from the axial center of the hole through plate 12. This angular displacement is sometimes necessary in order to accommodate the shape of the vertebra. On other occasions, it is simply a result of the imperfect drilling process. Whatever the cause, the angular displacement may necessitate a larger incision since the socket head of the removing device must be aligned with screw head 22.